This invention relates generally to electronic memories and particularly to electronic memories that use phase-change material.
Phase-change materials may exhibit at least two different states. The states may be called the amorphous and crystalline states. Transitions between these states may be selectively initiated. The states may be distinguished because the amorphous state generally exhibits higher resistivity than the crystalline state. The amorphous state involves a more disordered atomic structure. Generally, any phase-change material may be utilized. In some embodiments, however, thin-film chalcogenide alloy materials may be particularly suitable.
The phase-change may be induced reversibly. Therefore, the memory may change from the amorphous to the crystalline state and may revert back to the amorphous state thereafter, or vice versa, in response to temperature changes. In effect, each memory cell may be thought of as a programmable resistor that reversibly changes between higher and lower resistance states. The phase-change may be induced by resistive heating.
In some embodiments, the cell may have a large number of states. That is, because each state may be distinguished by its resistance, a number of resistance determined states may be possible, allowing the storage of multiple bits of data in a single cell.
A variety of phase-change alloys are known. Generally, chalcogenide alloys contain one or more elements from column VI of the periodic table. One particularly suitable group of alloys is GeSbTe alloys.
A phase-change material may be formed within a passage or pore through an insulator. The phase-change material may be coupled to upper and lower electrodes on either end of the pore.
Generally, the lower electrode is formed by a blanket deposition of an appropriate material. However, the use of a blanket deposition results in a lower electrode, extending across the length of the cell, that is capable of shunting the circuit and reducing the dynamic range of the memory cell. As a result, more current may be needed to heat the phase-change material to induce the phase-change.
Another problem relates to the adherence between the insulator defining the pore and the phase-change material. Because of the nature of these materials and the thermal cycling that they must endure, the adherence between the insulator and the phase-change material may be poor. One solution to this problem is to provide an interfacial layer that promotes adhesion between the insulator and the phase-change material. However, depositing the adhesion-promoting layer over silicon dioxide spacers may create adhesion problems as well. Therefore, the use of blanket deposition techniques to deposit the adhesion-promoting layer does not adequately promote adhesion of the phase-change material.
Thus, there is a need for better ways to deposit materials for forming phase-change memories.